As promised, Microsoft today delivered an emergency patch for a Windows Web server flaw that is being actively exploited by hackers.

The fix addresses a vulnerability in ASP.Net's encryption that attackers could abuse to access Web applications with full administrator rights; decrypt session cookies or other encrypted data on a remote server; and access and snatch files from sites or Web applications.

ASP.Net is the Microsoft-designed Web application framework used to craft millions of sites and applications.

Microsoft pegged the single bug addressed Tuesday as "important," the second-highest ranking in its four-step system. "Based on our comprehensive monitoring of the threat landscape, we have determined an out-of-band release is needed to protect customers, as we have seen limited attacks and continued attempts to bypass current defenses and workarounds," the company said yesterday.

Microsoft took the unprecedented step of releasing the update only to its download center, where customers must retrieve and install it manually. It won't push the patch to Windows Update until later.

"This is the first time we've released [an] update this way, but due to the nature of the active attacks and the severity of the potential loss of data, we are releasing the security update to the Microsoft Download Center first so customers (specifically large enterprises, hosting providers and ISVs) can begin updating their systems," Microsoft said in an e-mailed statement to the IDG News Service yesterday.

Security experts agreed that it was a smart move on Microsoft's part.

"While strange, it's not something that people should be worried about," said Andrew Storms, director of security operations at nCircle Security today. "The affected audience -- large companies, ISPs -- probably don't use Windows Update anyway. They likely have a much more stringent testing protocol."

Wolfgang Kandek, CTO of Qualys, a California-based security risk and compliance management provider, agreed that getting the patch out this way was acceptable. End users, he said, are not typically vulnerable to attack since few run a Web server.

"They may have been looking at another week or so of testing," Kandek said, referring to the process Microsoft goes through to make sure patches deploy properly via Windows Update. "And with attacks happening, this lets them get it into the hands of administrators who need it now."

Both Storms and Kandek remarked on Microsoft's patch pace in this case: Just 11 days from disclosure to update.

"The next closest was January of this year, when Microsoft patched the Google [Aurora] vulnerability," said Storms, speaking of the vulnerability in Internet Explorer that attackers used to break into the corporate networks of Google and more than 30 other major companies. "That was just seven days [from disclosure to patch], but after the fact we found out that Microsoft had known of the bug months earlier, so it's not a fair comparison."

It's possible that Juliano Rizzo and Thai Duong, the researchers who revealed the flaw Sept. 17 at an Argentinean security conference, had notified Microsoft prior to their presentation, said Kandek. If so, that would have given Microsoft more than the 11 days to fix the flaw.

Storms, however, thought otherwise. "Based on the fact that there is no name or names [crediting the vulnerability find] in the advisory tells me that Microsoft didn't know about it prior to Sept. 17," he said.

Customers who apply today's patch do not need to remove the workarounds they have applied at Microsoft's direction, a company spokesman said today. "Once you apply the security update, however, the workarounds are no longer required and can be removed," Dave Forstrom, the director of communications with the Microsoft Security Response Center (MSRC), said in an e-mail reply to questions.

Tuesday's rush update, called an "out-of-band" patch to show that it wasn't released in Microsoft's normal monthly schedule, was the fourth emergency fix so far this year.

Microsoft will release its next collection of security updates on Oct. 12.

The MS10-070 update can be downloaded from Microsoft's Download Center. It will land on Windows Update and the Windows Server Update Services (WSUS) in "the next few days," Microsoft said.